Refactor bootstrap dropdowns

While the bootstrap_btn_dropdown theme hook was "technically" converted, it really didn't undergo any real scrutiny.

http://getbootstrap.com/components/#dropdowns
http://getbootstrap.com/components/#btn-dropdowns
http://getbootstrap.com/javascript/#dropdowns

The following issues need improvement:

• Theme hook name is inaccurate. The dropdown isn't always triggered by a button.
• Label processing in general needs improvement. It's not always a link, it could be a button or a split button. It should also support custom icons
• Attribute/class processing.
• Link processing:
  • These links aren't even using the bootstrap_links theme hook, which is what it was created for (there's a lot of "dropdown" specific code in this theme hook). It should just be merged into this one.
  • Need option to right align menu.
  • Option processing:
    • A way to group links (using headers and dividers)
    • Control disabled state
    • Icon support